1. Field of the Invention
The invention relates in general to electrical fuses and in particular to thermally-activated, shorting-diode switches.
It is to be noted, however, that while the subject invention will be described with reference to particularized embodiments and end uses, the invention is not limited to such embodiments and uses. Those having ordinary skill in the art and access to the teachings of this specification will recognize additional implementations and utilizations within the invention's scope.
2. The Prior Art
In a number of situations where it is desired to provide a shorting function across any defective one of a plurality of series-connected battery cells, a critical requirement is that there be absolutely no interruption of circuit current flow during fuse operation. One such situation is a communications satellite in which stringent constraints often nedessitate system configurations such that even the briefest interruption in the current supplied to volatile processor circuitry would result in the irretrievable loss of basic operational capabilities. The constraints are a direct consequence of the inherently weight-limited nature of the typical satellite environment. Because the weight limitations preclude the use of supplemental current-maintenance devices, it is essential that the ultimately-employed battery-shorting mechanisms possess in themselves the needed continuous-current capability.
An unacceptable drawback of the prior diode-shorting fuses is that they provide no such absolute assurance against current interruption. Consider, for example, the device of U.S. Pat. No. 3,213,345. The fuse there includes an operationally-alterable lead component which must be physically displaced in order to establish the desired shorting contact. During the nonconductive, pre-shorting phase of device operation, the spring-like lead is held in electrical contact with a diode element by means of a temperature-sensitive solder bond. It is the interposed nature of the solder, together with the spring-induced, lateral shifting experienced by the lead, which creates the conceivable danger that during activation the device's circuit path will be momentarily interrupted. This danger would be intensified in the severe physical environment of a satellite where solder flow characteristics and lead displacement behavior could very well be altered by the extreme vibration, random orientation and centrifugal forces which are inherent aspects of typical flight profiles. As a result, the '345 device could not be employed where critical processing functions are at stake.
An unattrative alternative to such interruption-susceptible shorting devices is the continued utilization of conventional diode structures. While the non-operationally-alterable nature of such structures does provide an otherwise-acceptable degree of assurance against current interruption, their only "shorting" capability is by means of their low-resistance, forward-conduction state. It is again in the operationally-constrained spacecraft environment, especially as often compounded by the conventional, weight-intensive need to employ a multiplicity of diodes for battery-bypass purposes, that even the minimal power consumption associated with such low-resistance states necessitates equipment tradeofts resulting in unappealing restrictions on overall system capabilities.
It is apparent, therefore, that a need exists for diode switching devices having both absolute assurance against current interruption and a direct shorting capability.